1. Field of Endeavor
The invention relates to a premix burner for a heat generator, with partial cone shells which make up a vortex generator, and which encompass a conically widening vortex chamber and mutually define tangential air inlet slots, and also with feeds for gaseous and/or liquid fuel, of which at least one is arranged along the air inlet slots on the partial cone shells, and at least one other is arranged along a burner axis which centrally passes through the vortex chamber.
2. Brief Description of the Related Art
Generic type premix burners have effectively been used for many years for firing combustion chambers to drive gas turbine plants, and represent largely perfected components with regard to their burner characteristics. Depending upon application and desired burner capacity, generic type premix burners are available which are optimized both with regard to burner capacity and also from the point of view of reduced emission of pollutants.
Such a premix burner is to be gathered from EP 0 321 809 B1, which premix burner basically includes two hollow, conical body sections which fit one inside the other in the flow direction, the respective longitudinal symmetry axes of which extend in an offset manner to each other so that the adjacent walls of the body sections in their longitudinal extent form tangential slots for a combustion air flow. Liquid fuel is preferably injected through a central nozzle into the vortex chamber which is encompassed by the body sections, while gaseous fuel is introduced through the additional nozzles which are present in the region of the tangential air inlet slots in the longitudinal extent.
The burner concept of the aforementioned premix burner is based on the generation of a closed vortex flow inside the conically widening vortex chamber. The vortex flow, however, on account of the increasing vortex in the flow direction inside the vortex chamber, becomes unstable and changes into an annular vortex flow with a backflow zone in the flow core. The location at which the vortex flow changes into an annular vortex flow with a backflow zone by means of bursting is basically determined by the cone angle which is inscribed by the partial cone shells, and also by the slot width of the air inlet slots. In the case of the selection for dimensioning of the slot width and also of the cone angle, by which the overall length of the burner is ultimately determined, narrow limits are basically set so that a desired flow field is established, which leads to the formation of a vortex flow which in the burner mouth region bursts into an annular vortex flow, forming a spatially stable backflow zone, in which the fuel-air mixture ignites, forming a spatially stable flame. A reduction in size of the air inlet slots basically leads to an upstream shift of the backflow zone, as a result of which, however, the mixture of fuel and air then ignites temporally and spatially earlier.
On the other hand, in order to position the backflow zone which is formed further downstream, i.e., to obtain a longer premix path or evaporation path, a mixing path in the form of a mixer tube, which transmits the vortex flow, is provided downstream of the vortex generator, as it is described in detail, for example, in EP 0 704 657 B1. In this publication, a vortex generator which includes four partial cone bodies is to be gathered, to which vortex generator a mixing path, which serves for a further mixing-through of the fuel-air mixture, is connected downstream.
Transfer passages are provided for continuous transfer of the vortex flow which issues from the vortex generator into the mixing path, which transfer passages extend between the vortex generator and the mixing path in the flow direction and serve for the transfer of the vortex flow which is formed in the vortex generator into the mixing path which is connected downstream to the transfer passages.
In addition to the constructional burner design, the feed of liquid fuel also has a decisive influence on the flow dynamics of the vortex flow which is formed inside the vortex generator and also of the backflow zone which is formed as spatially stably as possible downstream of the vortex generator. In this way, with a typical feed of liquid fuel along the burner axis, a rich fuel-air mixture, which is formed along the burner axis, becomes apparent at the location of the cone apex of the conically widening vortex chamber, especially in premix burners of larger design, as a result of which the risk of the so-called backflash in the region of the vortex chamber increases. Such backflashes lead on the one hand inevitably to high NOx emissions, particularly through which fuel-air mixture portions which are not completely mixed through are combusted. On the other hand, backflash occurrences are hazardous especially because of this and are to be avoided since they can lead to thermal and also mechanical stresses and, as a consequence of this, can lead to irreversible damage to the structure of the premix burner.
By means of the burner design which is described above, which is adapted in an optimized manner to the desired burner conditions in each case, it is clear that by mere size scaling of all premix burner components to form a larger burner with larger capacity, the desired burner characteristics are not also automatically maintained. If, in this way, for example the mass flow of a gas turbine is not linearly scaled to the geometric scaling factor of individual gas turbine components, but is largely quadratic, i.e., the output is to be doubled by size adjustment of the gas turbine plant, it is necessary to provide four times as much air for the combustion process. This has the result that, for each individual gas turbine plant, which differs by size and power factor, a completely new burner, and especially a completely new premix burner, has to be designed and built, which it is necessary to adapt to the desired optimized burner characteristics in a suitable manner. This incurs high costs which it is necessary to avoid. A large number of individual burners are arranged in a circular arrangement around a combustion chamber especially in high-output gas turbine plants in order to achieve an optimum burner performance with regard to burner capacity and also pollutant emissions, depending upon gas turbine output. It is clear, moreover, that single-row burner arrangements, but especially double-row or multi-row burner arrangements, around in each case one combustion chamber, demand large constructional volumes.
The preceding embodiments show that a power output variation in the sense of a power output increase of a gas turbine plant by the currently known means inevitably necessitates a complete new construction of a hitherto known conically formed premix burner. In this case, it is necessary to take remedial action and to search for measures in order to enable a desired scaling of gas turbine plants also to the premix burners which are currently in operation, and with only minor structural changes to existing premix burner systems.